1. Field of the Invention
This invention relates to load positioning systems used for moving a load in a gravity field and particularly to apparatus for compensating for the gravity force in a manner permitting symmetrical load acceleration and velocity with low cost, weight and space penalties.
2. Description of the Prior Art
In load positioning systems it is known to compensate for the effects of gravity on the load by means of a counterbalance or counterweight, a constant-force spring, or a pneumatic equilibrator.
A counterweight is often undesirable due to its large size or to the weight/inertia that it adds to the system. For example, in a typical three degrees-of-motion system with X, Y and Z axes, the mass of the Z counterweight often degrades the performance of either or both of the X and Y drives, which leads to the necessity for more power. Additional power translates to more apparatus cost, weight, and volume.
In many situations a constant force spring is not capable of generating a sufficient compensating force. These springs have innately high stresses which induce metal fatigue. Consequently, the springs have a relatively short dynamic life span.
The disadvantage of pneumatic equilibrators is that they are long and bulky and generally have to be made on a custom basis.
In a load positioning system with a servomechanism it is known to bias the servo amplifier in an attempt to compensate for the force of gravity. This procedure is not optimum inasmuch as it only reduces the steady-state error. When the bias is added, the maximum acceleration and maximum velocity remain asymmetrical--that is, the acceleration and the velocity in the direction opposite the gravity vector are less than the acceleration and the velocity in the direction of the gravity vector. Motor selection must therefore be made according to the performance demands of the worst case. This results in greater apparatus cost, volume and weight.
A limited scale patent search on the subject invention uncovered these patents, not one of which even recognizes the problem solved by the present invention. U.S. Pat. Nos. 4,105,961; 4,200,828; and 4,301,528 disclose phonograph arms wherein motors are utilized to adjust styli forces and facilitate groove tracking. Gravity measuring devices are described in U.S. Pat. Nos. 3,408,868; 3,991,625; and 4,022,064. U.S. Pat. No. 3,854,378 discloses an apparatus for stabilizing an oscillating mass such as an artillery gun and U.S. Pat. No. 2,789,260 sets forth a device for electronically indicating deviation from the earth's gravity vector.